This project is aimed at identifying effective targets to address a major problem facing cancer treatment: bone marrow environment-mediated drug resistance (EMDR). One potential key target that may prevent/reduce EMDR is STATS, in part because it is persistenfiy activated in both tumor cells and in the tumor microenvironment, and because it is a transcription factor that regulates expression of numerous tumorpromoting and drug resistance factors. Many of these factors are also STATS activators, thereby forming a feed-foHA/ard loop linking tumor cells with the tumor microenvironment. lL-6, produced by bone marrow stromal cells has been shown to activate STATS in neuroblastoma cells (collaborative work with Project 1), leading to tumor growth and EMDR, Our preliminary studies show that ablafing STATS in bone marrow-derived cells prevents formation of lung niches that harbor metastatic tumor cells. However, due to a lack of intrinsic enzymatic activity, there are no clinical reagents to specifically target some of the key oncogenic transcription factors such as STATS. Our recent studies demonstrate that STATS upregulates expression of S1PR1, a key G protein-coupled receptor for the phospholipid metabolite, SIP. SI PRI, in turn, activates STATS through tyrosine kinase JAK2, which is also critical for IL-6 receptor signaling. Elevafing SI PRI expression in either tumor cells or bone marrow-derived myeloid cells leads to production of numerous factors that promote STATS-dependent niche formation to provide sanctuary for tumor cells. Furthermore, recent studies implicate SI PRI in mediating drug resistance of both solid and blood-borne cancers, including ALL. Importantly, both SI PRI and JAK2 inhibitors have recently entered clinical trials. We therefore propose to test the hypothesis that targeting S1PR1/JAK2/STAT3 signaling in bone marrow stromal cells will effectively inhibit EMDR for both neuroblastoma and ALL. We will test our hypothesis in co-cultures of neuroblastoma and ALL tumor cells with mouse and patient bone marrow stromal cells, and in syngeneic transgenic mouse and xenograft neuroblastoma/ALL models. We will also adapt a novel in vivo ectopic bone-forming system to reproduce the tumor/bone marrow niche (both mouse and human). With the availability of transgenic mice and SiRNA technologies allowing S1PR1, JAK2, or STAT3 knockout/knockdown in specific types of bone marrow cells as well as the availability of small molecule drugs currently under clinical tesfing, our proposed studies may lead to paradigm-shifting novel therapies to overcome EMDR in neuroblastoma, ALL, and other types of cancer.